The present invention relates to an electron source of a display unit for use in a television receiver, a terminal display of a computer, etc.
Conventionally, in an electron source employing a linear thermionic cathode, there has been a drawback that the cathode itself is vibrated during drive of the electron source, thereby resulting in a variation of the quantity of emitted electron beams. In order to prevent vibrations of the cathode, Japanese Laid-Open Patent Publication Nos. 63-187538 (1988) and 2-33838 (1990) and U.S. Pat. No. 4,887,000 propose a countermeasure in which a support member having a contact portion held in contact with a portion of the cathode is provided.
One example of a known electron source employing a linear thermionic cathode is described with reference to FIG. 1, hereinbelow. In FIG. 1, a striped back signal electrode 32 is formed on an insulating substrate 31. A plurality of linear thermionic cathodes 33 are provided in a direction perpendicular to the face of the drawing sheet of FIG. 1 so as to intersect with the back signal electrode 32. A grid electrode 34 is provided above and substantially in parallel with the cathodes 33. In order to prevent vibrations of each of the cathodes 33, a cathode support means 36 which is obtained by forming holes on a flat plate is provided such that the holes confront the back signal electrode 32. Hence, in FIG. 1, the cathode support means 36 has a plurality of wall portions 36A. The cathode support means 36 is constituted by a metallic substrate 36a formed with the holes by etching or the like and a heat resistant insulating member 36b coated on the metallic substrate 36a. Meanwhile, the cathode 33 is strained so as to be lightly pressed against the cathode support means 36. Although not specifically shown, all the above mentioned components of the known electron source are enclosed in a vacuum vessel,
The operation of the known electron source of the above described arrangement is described as follows. In FIG. 1, when a voltage which is positive relative to the cathode 33 is applied to the grid electrode 34 so as to heat the cathode 33 to temperatures enabling emission of electrons therefrom and a predetermined potential is imparted to the back signal electrode 32, the area in the vicinity of the cathode 33 corresponding to a portion of the back signal electrode 32 having the predetermined potential imparted thereto partially has an electric field in which electrons can be emitted from the cathode 33. As a result, an electron beam 35 is emitted from the cathode 33. The known electron source employing the linear thermionic cathode 33 described above is used for a picture display device disclosed in, for example, U.S. Pat. No. 31,876.
However, in the known electron source, surface of the insulating member 36b adjacent to its portion held in contact with the cathode 33 is electrically charged in a driving state of the cathode 33. Thus, even if a pair of neighboring sheet portions 32A of the back signal electrode 32, which interpose each of the wall portions 36A of the cathode support means 36, are set at a quite low potential, such a state is brought about in which electron beams are emitted at all times from portions of the cathode 33 held in contact with the cathode support means 36. Electron beams emitted from the portions of the cathode 33 held in contact with the cathode support member 36 pass partially, as leaked electron beams, through apertures of the grid electrode 34 as shown by the broken lines in FIG. 1. Accordingly, a switching voltage required for switching the electron beams on and off becomes extremely high.
In order to completely eliminate the leaked electron beams, a distance between the aperture of the grid electrode 34 and a portion of the insulating member 36b held in contact with the cathode 33, namely, actually a pitch of stripe of the back signal electrode 32 and a pitch of the apertures of the grid electrode 34 is required to be increased.
On the contrary, if electron beams should be finely controlled spatially without changing these pitches, it is impossible due to the leaked electron beams to create a state having no electron beam.